This invention relates to a method of producing polycrystalline diamond composites, and to polycrystalline diamond composites so produced.
Polycrystalline diamond (PCD) composites generally consist of a layer of polycrystalline diamond bonded onto a carbide substrate using a high pressure-high temperature (HpHT) process. The PCD layer is a dense layer of sintered diamond particles in a metallic binder phase, such as cobalt, iron, or nickel, or an alloy containing one or more such metals. The method for producing a PCD composite generally comprises encapsulating diamond powder and a carbide substrate in a refractory metal, such as tantalum, molybdenum and niobium, and subjecting the encapsulated assembly to typical HpHT conditions at which the polycrystalline diamond is crystallographically and thermodynamically stable. The source of the binder phase is generally provided by the carbide substrate, which infiltrates into the PCD layer during the HpHT process.
Polycrystalline diamond composites are preferred in a number of applications, such as oil and gas drilling, cutting tools for machining, and as wear parts. Fine grain PCD has displayed superior wear and abrasion resistance over coarser diamond grades due to the high strength and hardness of the finer diamond grain size PCD.
During the HpHT process, metal infiltrates the diamond layer involving the processes of melting, capillary action and diffusion; and acts as a solvent/catalyst for the recrystallisation and sintering of the diamond particles. During infiltration, the molten solvent metal from the carbide substrate reacts with the immediate diamond layer, dissolving the fine component of that layer completely.
Very fine grades of PCD, where the diamond particle size is typically less than 2 μm, are often prone to abnormal grain growth due to the much higher solubility of very fine diamond in the molten metal solvent/catalyst. Through a mechanism known as Ostwald ripening, carbon from the fine particles dissolves preferentially (compared to the coarser particles) in the solvent/catalyst; and then re-precipitates on any remaining coarse particles themselves. This size-dependent solubility effect results in the exaggerated growth of the coarse particles where abnormal diamond grains that are typically between 50 and 200 μm in size can be observed. Grains such as these constitute major flaws within the diamond microstructure and can significantly reduce the performance of the material.
Abnormal diamond grain growth is reported to be controlled by the use of sintering aids such as WC, Ni—Zr alloy and cubic boron nitride [1]. The use of W metal as a sintering aid to control grain growth is also described in U.S. Pat. No. 6,261,329.
U.S. Pat. No. 5,441,817 describes the use of a thin refractory shim or layer on the WC—Co substrate together with admixed refractory material within the diamond powder layer. A suitable refractory material is said to be titanium carbide and/or titanium carbonitride. These are said to regulate the flow of molten metal from the substrate into the diamond layer and hence minimize abnormal grain growth and bond metal depletion at the diamond/substrate interface.
One disadvantage of these prior art systems lies in the formation of complex intermetallics with the cobalt binder in the PCD layer, which rely on accurate control of HpHT conditions. Another disadvantage of the prior art systems is the interference of these additional species with the kinetic sintering processes. This is likely to influence diamond/diamond bonding and especially so for finer-grained (<2 μm) PCD.
U.S. Pat. No. 4,311,490 describes a polycrystalline diamond composite in which the PCD layer is produced from a layer of coarse diamond of particle size 75 to 500 μm adjacent the carbide substrate and a layer of fine diamond of particle size less than 10 μm on the layer of coarse diamond. The working example uses diamond of 6 μm particle size for the fine diamond. The use of coarse and fine diamond in a layered structure is said to reduce the incidence of soft spot formation in the PCD. The problem of abnormal grain growth is not addressed, which is not surprising since such abnormal grain growth would not be evident in the PCD of this US patent because of the relatively large diamond particles used in the “finer” layer.